Thank You ryecoaaron.
My mistake was, that the Data-Raid-Partition was mounted to the Folder /data.
OMV mounts in /media/....
Unmount my Partiton an Mount it again in OMV solved the Problem.
I know, that a separate System Disk ist the Way OMV wants, but my Atom-Board has only 4 SATA-Ports and I've 4 Drives installed.
Further I don't like to use a USB-Device as Systemdrive.
I use this configuration a lot of years - and You know .. You can't teach an old dog ........
Thanks for Your help.
Best regards
Robert